Currently, WALSegmentIterator implementations read to the end of
the end of their list of segments and return EOF. This commit adds
the ability to push additional segments to in-process iterators and
notify their callers that new segments are available. This is only
implemented for the file-based iterator but other segment iterators
may get this implementation in the future or have a wrapping
iterator provide a polling-based implementation.
Previously, the index & offsets were encoded as 8-character hex
strings, however, this limits the maximum value to a `uint32`. This
is normally not an issue, however, indices could go over the maximum
value of 4 billion over time and the offset could exceed this value
for an especially large WAL update. For safety, these encodings have
been updated to 16-character hex encodings.
This commit refactors out the complexity of downloading ordered WAL
files in parallel to a type called `WALDownloader`. This makes it
easier to test the restore separately from the download.
